A significant challenge in semiconductor fabrication is to etch away an oxide layer, or a portion of the oxide layer, which has been placed over a nitride layer such as silicon nitride. The challenge is to perform the etch with high selectivity for the oxide layer. Because both oxide and nitride materials generally etch at about the same rate in an etch plasma, the challenge of providing a process with sufficient selectivity has not been met.
One attempt to meet this challenge uses a fluorine-substituted hydrocarbon as an etchant. The fluorine reacts with the carbon present to form a passivating coating of a carbon-fluorine polymer over the substrate. This polymer is dissociated by oxygen atoms formed during the etch of the oxide layer. Thus, as the oxide layer is continually etched, the nitride layer etches at a much slower rate due to the presence of the passivating coating. Unfortunately, the passivating layer is also attacked by free fluorine atoms present in the plasma and, as a result, etching of the nitride layer continues at a rate greater than desired.
Marks et al. disclose, in U.S. Pat. No. 5,423,945, another process for selectively etching an oxide layer over a nitride layer. The disclosed process uses highly polymerizing gases, such as C.sub.4 F.sub.8 or CH.sub.2 F.sub.2, with a fluorine scavenger, such as CO, to form a protective carbon-rich polymer layer on the nitride layer surface. Although enhanced etch selectivity of the oxide layer to the nitride layer is achieved, several drawbacks occur using this process.
First, the process disclosed by Marks et al. requires the buildup of polymer on the nitride layer surface to slow down the etch rate on the nitride layer. This process is not ideal for very thin nitride layers of 200 angstroms or less because the nitride layer may be consumed by etching before the selectivity effect is achieved. Second, the use of a polymerizing gas is not desirable because its high selectivity is likely to perpetuate defects during subsequent etching of the nitride layer, a step conventionally conducted in semiconductor processing.
In addition, highly polymerizing gases tend to leave substantial residue both on the substrate being etched and in the process chamber. As a result of this residue, both the substrate and the process chamber typically require a post-process clean. Furthermore, many polymerizing gases, including CH.sub.2 F.sub.2, are flammable, toxic, or both flammable and toxic. Similarly, CO, which is used as a scavenger gas, is toxic and tends to leach nickel and iron from stainless steel components in the gas line of the etching chamber apparatus.
The deficiencies of the use of conventional processes for etching an oxide layer over a nitride layer show that a need exists for a process that has a high selectivity for the oxide layer, uses non-polymerizing gases, and is suitable for use on an oxide layer disposed on a thin nitride layer. To overcome the shortcomings of conventional processes, a new process is provided. An object of the present invention is to provide a process for etching an oxide layer over a nitride layer with high selectivity. A related object is to provide a process suitable for oxide layers disposed on thin nitride layers. Still another object of the present invention is to provide a process that uses non-polymerizing gases.